September 18: With binoculars begin looking for Vesta (4 Vesta, m = 6.4), moving eastward among the stars of Sagittarius. In a week it passes south of Saturn. Tonight, the minor planet is 1.3° to the lower right of 9 Sagittarii (9 Sag, m = 5.9), a star in the central area of the Lagoon Nebula (M8, NGC 6523).
September 16: The moon reaches its First Quarter phase, 6:15 p.m. CDT. Venus (m = −4.8) begins its period of greatest brightness, a two-week interval where it displays its greatest visual brightness. On September 21, Venus is at its greatest illuminated extent. Many of us know this time as “greatest brilliancy.” The two events are about a half-day apart. The instance of greatest brilliance is nearly impossible to see; rather, I note the time period when the planet displays its greatest visual brightness. The hundredths of a magnitude that distinguish greatest brilliancy are imperceptible to our eyes. Venus has an elongation of 40° ‒ midway between greatest elongation and inferior conjunction. Through a telescope it has an evening crescent phase with a 25% illumination and a 40” apparent size. With these factors Venus presents to us an illuminated phase that covers more area of the sky than any other time during its apparition and it is at its brightest. (For a more technical explanation of greatest illuminated extent, see https://tinyurl.com/venus-greatest-illuminated.)
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